a) Field of the Invention
This invention relates to a motor, in particular a spindle motor having a magnetic fluid seal mechanism and more particularly to a magnetic ring constituting a part of a magnetic fluid seal mechanism.
b) Description of the Related Art
Among spindle motors for disc drives having a magnetic fluid seal mechanism for blocking leakage of dirty air and oil dust from a ball bearing and the like present in a space inside the spindle motor to an outside space, one such construction is shown in U.S. Pat. No. 5,047,677.
In this disclosure, a magnetic fluid seal mechanism 3 is mounted on the inner peripheral surface of the projecting wall portion of the hub 6. The magnetic fluid seal mechanism 3 is made up of a magnet unit mounted on the inner peripheral surface of the projecting wall portion of the hub 6, a magnetic ring 2 fixed to the outer peripheral surface of the fixed shaft 1a above the inner ring of the ball bearing 4 and made of a magnetic material, and magnetic fluids 3c held in a gap between the outer peripheral surface of the magnetic ring 2 and the magnet unit. The magnet unit is made up of a ringlike magnet 3a, a ringlike pole piece 3b mounted on the inner peripheral surface of the hub 6 and covering the upper and lower end surface of the magnet 3a. The pole pieces 3b project further toward the magnetic ring 2 than the inner peripheral surface of the magnet 3a.
The magnetic ring 2 used in the magnetic fluid seal mechanism 3 is conventionally made by cutting a bar to a predetermined length, cutting a hole in the bar to make a ring, adjusting the internal diameter by passing a reamer through this hole and grinding the inner peripheral surface. However, with this kind of process, not only is the time required to manufacture one magnetic ring long and the machining cost high but also, because a hole is made in the center of a bar, the material of the portion cut away has been wasted. Furthermore, when the magnetic ring 2 has been made by cutting, burrs and the like have been produced on the inner peripheral surface of the magnetic ring 2 and it has been difficult to increase the cylindricality of the magnetic ring 2 and to make it with good accuracy.